The present invention relates to a universal serial bus (USB) device and related method, and more particularly to a charging control circuit of the USB device and related method.
A portable device with a universal serial bus (USB) port, e.g., USB 2.0 port, is capable of charging a battery in the portable device via the USB 2.0 port when the battery of the portable device is under a dead battery or a weak battery condition. When the portable device with the dead battery or the weak battery condition is connected to a downstream port, the portable device is allowed to draw an external current from the downstream port to charge the battery. The portable device is also required to assert a predetermined voltage at a first I/O (Input/output) pin of the USB 2.0 port within a first predetermined time after the external current is higher than a predetermined magnitude of current. Then, the predetermined voltage remains asserted after the external current drawn from the downstream port is higher than the predetermined magnitude of current. When the portable device is ready to be connected, the predetermined voltage at the first I/O pin of the USB 2.0 port should be disabled, and a predetermined current should be enabled on a second I/O (Input/output) pin of the USB 2.0 port, lasting for a predetermined time interval in order to check if the data pins (i.e., the first and second I/O pins) have made contact with the host. In other words, during the procedure of charging the battery in the portable device via the USB 2.0 port, a predetermined voltage should be asserted at the first I/O pin of the USB 2.0 port for a first predetermined time interval, and a predetermined current should be asserted at the second I/O pin of the USB 2.0 port for a second predetermined time interval. However, due to the fact that the core circuit and the related system of the portable device may not be ready when the battery of the portable device is under the dead battery or the weak battery condition, the voltage at the first I/O pin of the USB 2.0 port is hard to maintain at the predetermined voltage during the charging procedure. Similarly, the current at the second I/O pin of the USB 2.0 port is also hard to maintain at the predetermined current during the charging procedure. Providing an efficient and low cost mechanism to provide a predetermined voltage and a predetermined current at the data pins of the USB 2.0 port during the charging procedure is a significant concern in the field of USB portable devices.